Final answer:
Crossing over is a genetic process that occurs during prophase I of meiosis I, where homologous chromosomes exchange segments of DNA, leading to genetically unique recombinant chromosomes and increasing genetic diversity.
Step-by-step explanation:
Crossing Over in Meiosis
Crossing over is a pivotal event during meiosis that contributes to the genetic variation in offspring. It occurs specifically during prophase I of meiosis I. This process involves homologous chromosomes—chromosomes that are similar in shape, size, and genetic content—pairing up tightly in a formation called a tetrad. While in this formation, recombination occurs as sections of DNA are exchanged between non-sister chromatids, creating recombinant chromosomes. Each resulting chromosome contains a unique mix of paternal and maternal genetic information. This exchange of genetic material can happen multiple times within the same pair of homologous chromosomes, leading to new combinations of alleles and therefore contributing to the genetic diversity among sexually reproducing organisms.
Through crossing over, chromatids held together by a centromere become genetically distinct, which, after the completion of meiosis, gives rise to gametes with different combinations of alleles. This genetic reshuffling is instrumental for evolution and adaptation as it increases the genetic pool within a population.